This invention relates to a method for the beneficiation of mineral ores. More particularly, the invention relates to the beneficiation of phosphate ores containing alkaline earth metal carbonate mineral impurities.
Apatite is a common mineral and appears in small amounts in practically all igneous rocks (the term apatite as used herein is intended to include the mineral known as carbonate fluorapatite). Concentrations rich enough to justify mining are found in many localities. The mineral apatite is a phosphate of lime containing varying amounts of chlorine, fluorine, carbonate and hydroxyl. The phosphorus pentoxide content of various apatites ranges from 32 to 42 percent. The fluorine content has ranged as high as 3.8 percent, but generally is about 3.3 percent in fluorapatite.
The term "bone phosphate of lime", commonly abbreviated to BPL, is generally used to express the phosphate content of fertilizers. It is the equivalent of Ca.sub.3 (PO.sub.4).sub.2.
In the analysis of phosphatic materials, the chemist generally reports the phosphorus content in terms of phosphorus pentoxide (P.sub.2 O.sub.5).
For the major uses of apatite, the mineral is preferably in concentrated form. The phosphate industry requires, for the production of fertilizers, superphosphate, triple superhosphate and phosphoric acid, a phosphatic material of relatively high BPL content and imposes price pentalities where impurities are present in excess of certain maximum fixed percentages.
In order to be attractive on a commercial scale, a process for beneficiating a phosphate ore should produce a phosphate concentrate which is substantially free of gangue minerals. Many methods have been devised to beneficiate phosphate ores. Froth flotation beneficiation of phosphate minerals is commercially practiced on phosphate ores in which silicate minerals are the predominant gangue.
Such beneficiation generally comprises comminuting and classifying into various particle sizes. Coarser fractions may be suitable for direct sale or may be further beneficiated by sizing and skin flotation techniques. Extremely fine material, e.g. -325 mesh, which primarily contains clay slimes, is usually discarded. The intermediate fraction having a particle size range of -20 .+-.325 mesh poses the greatest beneficiation problems.
The "Crago" or "double float" froth flotation processes, as described by A. Crago in U.S. Pat. No. 2,293,640, Aug. 18, 1942, is commercially used for beneficiating such fractions of phosphate ores in which siliceous minerals are the predominant gangue. That process consists of conditioning the material with fatty acid reagents, flotation of the phosphate mineral, deoiling with sulfuric acid to remove the reagents, and refloating with amine reagents to remove the siliceous gangue which either floated or was trapped in the rougher fatty acid flotation.
Some phosphate ores contain carbonate gangue materials in addition to siliceous minerals. Alkaline earth metal carbonate minerals are common impurities in certain ore deposits. Examples of these deposits are the south Florida deposits and the western phosphates found in Idaho, Montana, Utah and Wyoming. Such mineral impurities include calcite (CaCO.sub.3), dolomite (Ca,MgCO3), sea shells, aragonite, dolomitic limestone, and other less common minerals. The "double float" process has generally been ineffective for beneficiating such ores, because the flotation characteristics of the carbonate minerals are very similar to those of apatite.
Snow, R. E., U.S. Pat. No. 3,259,242, July 5, 1966, teaches the beneficiation of calcitic-apatite ores in which the apatite is in the crystalline form. The method has not, however, been found entirely satisfactory for sedimentary deposits of ores containing oolitic or non-crystalline apatite or for dolomitic-apatite ores.
In copending application Ser. No. 788,070 filed Apr. 18, 1977, Snow teaches a flotation method for beneficiating a finely divided phosphate ore concentrate which is substantially free of siliceous gangue. Such method is very useful for beneficiating sedimentary phosphate ore concentrates. The present invention teaches the entire upgrading of a mineral ore matrix as mined from the earth. In addition to phosphate values, such a matrix typically contains clay slimes, siliceous gangue and alkaline earth metal carbonate mineral impurities. The matrix comprises particle sizes ranging from large rocks to very fine slimes. Heretofore, no satisfactory method has been discovered for recovering phosphate values in good yield and satisfactory purity from such a matrix.